Acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) currently kill the majority of afflicted patients despite treatment with combination chemotherapy and hematopoietic cell transplantation (HCT). The option of HCT for potential therapy of acute leukemias must be further extended to patients who do not have a readily available HLA-matched donor, such as patients in ethnic minority groups. Radiolabeled anti-CD45 monoclonal antibodies (Ab) have been shown to improve outcomes for AML and MDS in the setting of HCT, but toxicity remains high and cure rates are suboptimal. The objective of this research proposal is to develop a strategy to improve the cure rate of AML and MDS using radioimmunotherapy (RIT) pretargeted to the CD45 cell antigen. In Aim 1 we will optimize the therapeutic efficacy and toxicities of the pretargeted RIT approach by comparing the relative merits of 90Y- and 177Lu-labeled biotin in comparative biodistribution, dosimetry and therapy experiments to determine if the shorter path length b emissions of 177Lu afford more favorable tumor-to-normal organ ratios than those achievable with 90Y. In Aim 2 we will assess the relative merits of HCT employing MHC-haploidentical stem cells utilizing myeloablative pretargeted RIT with an anti-CD45 Ab (30F11)-streptavidin (SA) conjugate followed by either 90Y- or 177Lu-labeled DOTA-biotin (as determined from aim 1 the best radionuclide will be used), compared to conventional RIT using a directly radiolabeled anti-CD45 Ab (30F11) in clinically relevant disseminated AML murine leukemia model in which both leukemic cells and normal hematopoietic cells express CD45. We anticipate that the results from this aim will demonstrate that pretargeted RIT is superior to conventional RIT and will allow us to improve the therapeutic efficacy of haploidentical BMT, with tolerable toxicity. In Aim 3 we will characterize and maximize the myelosuppressive and immunosuppressive effects of radiation delivered to lymphohematopoietic tissues via either 90Y- or 177Lu-labeled biotin (as determined from aim 1) in combination with optimized supplemental doses of total body irradiation (TBI) and Fludarabine (FLU) in a preclinical murine haploidentical HCT model employing cyclophosphamide (CY) post-transplant graft-vs-host disease prophylaxis. Reducing the TBI and FLU doses, while administering high doses of pretargeted 90Y- or 177Lu-biotin as part of a preparative regimen for marrow HCT, would depend upon the demonstration of the ability of such an approach to: 1) ablate the marrow space, and 2) produce adequate immunosuppression. Thus, in aim 3 we will also evaluate the kinetics and durability of hematopoietic and immune cell reconstitution using an anti-mCD45 Ab-SA conjugate (30F11 Ab-SA) and radiobiotin, followed by reduced doses of TBI and/or FLU and infusion of MHC-haploidentical BM and post-transplantation CY in a murine leukemia model. We hypothesize that the pretargeted RIT strategy defined in this proposal will amplify the amount of radiation delivered to leukemia cells, decrease the radiation delivered to the liver, lungs, and other normal organs, improve remission and cure rates, prolong survival, and markedly attenuate toxicities compared to conventional RIT combined with standard conditioning reagents. We therefore anticipate rapid translation of the optimized promising pretargeted RIT into our clinical RIT HCT program for AML and MDS.